how to find out breed?

I was keeping a dog for friend now she is mine.She is suppose to be a weim-mastiff mix.Ilet her live in my house with elvis -year old bulldog- plot mix.i was having to chain her[this really hurt me]when i left so i'd have curtains when i got back but the bad weather we are having in s.e. N.C.{snow-lot of rain] istarted leaving her inside[closed curtain]so far so good.i noticed when she was on the chain and i was iside she wouuld play with other dogs that came by but let me come out and she would want to attack them or i let her out {not knowing they were out there]she tears them up] is this natural ?is she jealous ?elvis is a male and one is a female blood hound. ALSO how can I tell what she really is [she is black shiny slick coat  brown eyes inside of ears feel extremly slick wiegh about 100 lbs. 10 months old] I was glad to help my friend now i am glad  she gave her to me.SHE IS VERY LOVING and likes to sit behind me in the chairs.Elvis doesn't mind but he doesn't want her in the bed with us.I bought her a used sofa and put it in the bedroom so she could be near and comtable. you guessed it by now i love and care for my dogs like children.

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